Dangers of Mains Electricity Flashcards
All 7 cards in this deck
What is the function of the earth wire in an appliance with a metal case?
It is a low-resistance safety wire connecting the metal case to earth, so a faulty live case cannot stay at a high potential.
What is the function of a fuse in a mains circuit?
A thin wire that melts and breaks the circuit if the current gets too large.
What are the steps by which the earth wire and fuse make an appliance safe?
e.g. a loose live wire touches the metal case
- The case becomes live and a large current flows through the earth wire to earth.
- This large current melts the fuse in the live wire.
- The circuit breaks, disconnecting the appliance from the live supply, so the case is safe to touch.
In which wire must switches and fuses be fitted, and what does this achieve?
In the live wire — so that when the switch is off or the fuse blows, the appliance is cut off from the 230 V potential and is safe.
What happens if a connection is made between the live wire and earth?
e.g. a person touches a bare live wire
A large current flows through the connection to earth — causing an electric shock, burns, fire, or death.
What are the steps to choose a suitable fuse rating for an appliance?
e.g. an appliance that normally draws 3 A
- Find the normal working current: 3 A.
- Choose the next standard fuse rating above it (3 A, 5 A, 13 A): 5 A.
- Fit the fuse in the live wire.
True or false? A circuit breaker must be replaced after it has switched off the circuit.
False. A circuit breaker can be reset and used again, unlike a fuse, whose wire melts and must be replaced.